A lithium-ion secondary battery has characteristics that it has a higher energy density and is operable at a high voltage compared with other secondary batteries. Therefore, it is used for information devices such as cellular phones, as a secondary battery that can be easily reduced in size and weight, and nowadays there is also an increasing demand for the lithium-ion secondary battery to be used as a power source for large-scale apparatuses such as electric vehicles and hybrid vehicles.
The lithium-ion secondary battery comprises a cathode layer and an anode layer, and an electrolyte layer disposed between them. As the electrolyte layer, a non-aqueous liquid (electrolytic solution) and the like is employed.
As a technique related to such a lithium-ion secondary battery, Patent Document 1 for example discloses an anode active material used for a non aqueous lithium-ion secondary battery, the anode active material including a lithium-titanium compound oxide having a spinel structure, represented by a general formula of LiM0.5Ti1.5O4 (M is one or more element (s) selected from the group consisting of Fe, Co, Ni, Mn and Zn), and absorbing and releasing lithium ions, and a lithium-ion secondary battery prepared with the anode active material. Patent Document 2 discloses an anode active material of a lithium-ion secondary battery that is a single phase powder particle of Si including 0.01% or more to 0.40% by mass of B dissolved in solid. Patent Document 3 discloses a lithium-ion secondary battery comprising an anode active material, a cathode active material and a non-aqueous electrolyte, wherein a polycrystalline mesocarbon microsphere-graphitized article having a plurality of crystals and an average particle diameter of 1 to 120 μm, with the C axis direction of each crystal being in random directions is used as the anode active material. Patent Document 4 discloses a lithium-ion secondary battery prepared with an anode active material having a layered lithium iron nitride that is represented by a composition formula of Li3-xFexN (0<x<0.4).
On the other hand, graphite is known as a typical anode active material of a lithium-ion secondary battery, however, in a sodium ion secondary battery in which sodium ions move between a cathode layer and an anode layer, sodium ions are not inserted or removed in between graphite layers. Therefore, graphite cannot be used as an anode active material of a sodium ion secondary battery. Nowadays, it has been found that a hard carbon functions as an anode active material for a sodium ion secondary battery, therefore research and development of the sodium ion secondary battery has been intensely carried out.
As a technique related to such a sodium ion secondary battery, Patent Document 5 for example discloses a non-aqueous electrolyte secondary battery comprising a cathode having a complex oxide that includes sodium and iron as a cathode active material, an anode having a carbon material as a absorbing material for sodium ions and lithium ions, and a non-aqueous electrolyte containing lithium ions and sodium ions. Also, Patent Document 6 discloses a technique related to an anode active material for a sodium ion secondary battery that contains a carbon material and a sodium ion secondary battery in which the anode active material is used in an anode.